There are casual fans - people who just kinda like the show, like you would any good show. And then there are Trekkies. Trekkies come in three categories, in my opinion: 1. People who believe in the ideals and values of some aspect of the show on a religious or almost religious level, 2. People who are obsessed with the minutae of the show, and usually, canon, and 3. People who find the stuff of the show - phasers, uniforms, Klingon foreheads - to be their playground of choice.

There's some overlap between the categories, obviously, but I think most of us fall mainly in one of those three. I'm pretty strongly the first type, with a touch of the third (I own a uniform and a couple of pins), and a tiny bit of the second.